Which subunit of G proteins can bind GDP and GTP?

The alpha subunit of G proteins is responsible for binding GDP (guanosine diphosphate) and GTP (guanosine triphosphate). G proteins are heterotrimeric proteins composed of three different subunits: alpha, beta, and gamma. When inactive, the G protein is bound to GDP. Upon activation by a receptor, the alpha subunit exchanges GDP for GTP, leading to conformational changes that allow the G protein to interact with downstream effectors in signaling pathways.

Once GTP is bound to the alpha subunit, it becomes active and can dissociate from the beta and gamma subunits to initiate various cellular responses. The intrinsic GTPase activity of the alpha subunit eventually hydrolyzes GTP back to GDP, returning the protein to its inactive state.

The other subunits—beta and gamma—do not have the ability to bind GDP or GTP; instead, they play roles in stabilizing the inactive form of the G protein and can influence signaling pathways when in complex with the alpha subunit. The delta subunit is not part of the canonical G protein structure typically discussed in this context. Thus, the alpha subunit is uniquely responsible for GDP/GTP binding within the G protein framework